Prof. Chien-Shiung Wu – Nuclear Researcher

Prof. Chien-Shiung Wu

“These were moments of exhilaration and ecstasy! A glimpse of this wonder can be the reward of a lifetime. Could it be that excitement and ennobling feelings like these have kept us scientists marching forward forever?” -Chien-Shiung Wu on her discovery of parity violation in the weak interaction

– Graduate level study at the Institute of Physics of the Academia Sinica [1][2]

Ph.D: Nuclear Physics, Berkley [1][2]

Occupation: Nuclear physicist and research professor

Who is Chien-Shiung Wu?

Chien-Shiung Wu was a Chinese-American nuclear physicist known for being responsible for many major breakthroughs in physics, such as discovering parity violation in the weak interaction, and confirming the violation of charge conjugation in the weak interaction. She is also known for being the leader of the team that developed the process for separating uranium metal into uranium-235 and uranium-238, a problem Enrico Fermi, the so-called “architect of the nuclear age” could not solve.[6]

Life

Chien-Shiung Wu was born in Taicang, Jiangsu province in China in 1912, and was named using the family custom of having Chien being the first character of their forename followed by the characters “Ying-Shiung-Hao-Jie” which translate to the phrase “heroes and outstanding figures”, with her older and younger brother being named Chien-Ying and Chien Hao. [1] Wu would go on to prove this phrase true, as she became an outstanding figure in particle and nuclear physics due to her discoveries.

As a young girl, her father encouraged her in her studies and bought her three books for her self-study one summer on trigonometry, algebra, and geometry. This experience was the beginning of her habit of self-study and gave her sufficient confidence to major in mathematics in the fall of 1930. [2]

She graduated top of her class at “Suzhou Women’s Normal School No. 2” in 1929 and went on to study at National Central University from 1930 to 1934, studying mathematics at first but later moving to study physics, and after graduating she went on to study physics at postgraduate level at Zheijiang University, then becoming a researcher at the Insitute of Physics of the Academia Sinica. She and a friend of hers, Dong Ruo-Fen (a chemist from Taicang) left for the United States, where Wu had been accepted for a PhD place at the University of Michigan. Here she met and later married Luke Chia-Liu Yuan, another Chinese-American physicist who was the grandson of Yuan Shikai, the first president of the Republic of China. Upon hearing thatwomen who studied at the University of Michigan weren’t allowed to use the front entrance to the university, she decided to carry out her research at the University of Berkeley instead, where due to her brilliance she was immediately offered a PhD place even though the academic year had already commenced. [1][6]

Career and Research

Wu’s PhD thesis consisted of two parts: the first section was on the phenomenon of bremsstrahlung or “braking radiation”, that is, the deceleration of a charged particle when it is deflected by another charged particle (for example, an electron being deflected while moving close to an atomic nucleus) causing this charged particle to lose kinetic energy. This kinetic energy is converted into radiation in the form of a gamma photon, which is released from the charged particle.

The second section was on how radioactive isotopes of xenon are produced during the nuclear fission of uranium using the cyclotrons (particle accelerators which accelerate particles in a spiral trajectory) at the Lawrence Berkeley National Laboratory. [6]

In her research after World-War II, Wu carried on her investigations of beta-decay, a process that she had become fascinated with and was very fond of, saying “Beta decay was…like a dear old friend. There would always be a special place in my heart reserved especially for it”.[2]

In 1956, Chien led a group of experimentalists who tested whether parity symmetry occurred in the beta-decay of cobalt-60 nuclei. This was done by cooling cobalt-60 atoms to a temperature close to absolute zero and aligning the magnetic moments of the cobalt-60 atoms using a uniform magnetic field. As cobalt-60 is an unstable isotope, it decays to the stable isotope nickel-60 by beta-minus decay. The nickel-60 nucleus, however, is an excited state (a state consisting of a higher energy level), and decays promptly to its ground state by releasing two gamma photons. Because of the release of these photons, we know that this is an electromagnetic interaction, which is an important fact because EM interactions were known to conserve parity. [3]

What is meant by parity is as follows: it relates to the parity operator (P) which equates to the inversion of the three spatial coordinate axes, much like an object being reflected in the mirror. As shown in the figure below, the two clocks in the top row show parity symmetry, as the clock on left will act like the mirror image of the same clock (shown on the right). The hands will move in the opposite direction, but both clocks will show the same time. However, the two clocks on the bottom row are parity asymmetric, as the clock in the mirror image does not act in the same way as the clock on the left: the hands will not show the same time.[3]

The clocks in the top row behave act in the same way when reflected in a mirror, you can say that they conserve parity. The clocks in the bottom row do not, and you can say that they violate parity.

Conserve ParityViolate Parity

The Wu experiment used the fact that the gamma photon emission obeyed parity conservation as a control variable: if the emission of the electrons (a weak interaction process) was in the same direction as that of the gamma photons (an EM interaction, therefore conserving parity), we would know that the weak interaction also obeyed parity conservation, however, if the electrons were emitted in the opposite direction, we would know that the weak interaction violated parity. The latter was shown to be the case, hence firmly establishing that the weak interaction violated parity.

This discovery was a massive contribution to the development of the Standard Model of particle physics, however while her colleagues Tsung-Dao Lee and Chen Ning Yang were given the Nobel Prize in 1957 for their theoretical contributions towards this investigation, Chien, who was responsible for the experimental work leading to this discovery in the first place, wasn’t publicly acknowledged until she won the Wolf Prize in 1978. [1][6]

Later, in 1963, Wu investigated whether “charge conjugation” (a transformation which is responsible for “switching” particles into their anti-particles) also violated parity in the weak interaction in a series of experiments involving double-beta decay, and showed this to be true as well! She also performed experiments on pairs of entangled photons, and also on magnetism, researching the Mössbauer effect. Her experimental talents also reached beyond the field of physics, as she conducted research into the molecular changes in the deformation of hemoglobins that cause sickle-cell disease. Due to her massive impact in experimental research in the field of nuclear physics and beyond she was awarded many honors, awards and distinctions, including the National Medal of Science in 1975, being the first living scientist to have an asteroid named after her (2752 Wu Chien-Shiung).[1][5][8]

In 1981, Wu retired to become professor emerita, a title which is bestowed only upon those who have retired having had careers showing an outstanding service to their field of study.

Hardships faced due to racism.

When applying to Berkley for a PhD, Wu was taken to see the chairman of the physics department, R. Birge. Birge had been criticized as “prickly, narrow-minded, and prejudiced against foreigners, particularly Chinese, women, and anyone with a foreign accent.”, and even though Wu was accepted onto the PhD program it is upsetting to know she had to come into contact with, and have whether she got into the program decided by a vile, racist man. [2]

After the first year of her PhD was completed, Wu applied for a scholarship in order to no longer be financially provided for by her uncle. At the time the US discriminated against Asians. The chairman of the physics department, Birge, did not want to raise the ire of the Board of Trustees, and agreed to award Wu’s readerships with smaller stipends. Wu was yet again the target of systematic racism, and even though academically she outshone her white colleagues she was punished for simply being a different race, a fact that is incredibly disturbing.[2][6]

In 1944, Wu went to Columbia as a senior scientist, however, she was never promoted into having a teaching position. This was mainly due to I. I. Rabi, the most powerful member of the Columbia physics department, being an “old-school physicist”, with a stubborn opinion of women and Asians. Wu was often called “Chinese Madame Curie” as both Chien-Shiung Wu and Marie Curie worked on nuclear decay and fission, however, this is also racist as Wu shouldn’t be compared to a western female physicist in order for her work to be valued; and Wu herself didn’t feel like the phrase was an honour: “people think that calling me the ‘Chinese Madame Curie’ is an endorsement and honor, but I do not quite feel that way”.[2]

Chien-Shiung Wu was a remarkable physicist; one who was responsible for many incredible advancements in nuclear and particle physics, made even more incredible by the fact she achieved them whilst facing continued hardships due to the political climate and racist professors and colleagues whom she should have been able to rely upon. However, she eclipsed all of them due to her amazing talents and did this all without a hint of egotism: “This small, modest woman was powerful enough to do what armies can never accomplish: she helped destroy a law of nature. And laws of nature, by their very definition, should be constant, continuous, immutable, and indestructible.”[7]